ASI178MM-cooled: narrowband imaging exposure settings

Now up to recently, I had been choosing settings for imaging with the ASI178MM-cooled based on convenience. But now that I am getting familiar with imaging with narrowband filters, I have decided to take a closer look at the histograms to help me choose the most ideal gain and exposure settings for my combination of equipment.Equipment used:

ZWO ASI178MM-cooled camera @ -25 degrees C, Astronomik 12nm SII narrowband filter, Samyang 135mm F2.0 EOS mount lens @ F2.0, iOptron ZEQ25, Lacerta MGEN autoguider on Finder scope, Nebulosity 4.0 for image capture.


Seagull nebula IC2177

Ist image: Gain 100 5 min sub. This setting shows that there is no clipping of the left, and minimal clipping of highlights on the right. If I had the patience, this would be the ideal setting; but that would mean 3 x 20 subs of 5 min = 300mins (or 5 hours) for 1 DSO.

Gain 100 5 min sub
Gain 100 5 min sub

2nd image: Gain 200 3 min sub. This setting shows that the curve is not clipped on the left, and still minimal clipping on the right of the highlights. This is the setting I am likely to use as it requires 3 x 20 subs of 3 mins = 180 mins (or 3 hours) for 1 DSO. The noise is low enough to yield a very nice image after processing.

Gain 200 3min sub
Gain 200 3min sub

3rd image: Gain 300 2 min sub. This setting allows for the lowest read noise and high gain. There is more clipping of the highlights at this setting. It would mean that it requires 120 mins (or 2 hours) to capture 20 x 2min subs for each narrowband channel. But that is at the expense of more noise. Hence I’ve gone away from imaging at this gain setting, unless I don’t have a choice.

Gain 300 2 min sub
Gain 300 2 min sub

Just to put things in perspective, as the images are captured at F2.0, the exposure equivalent of a 3-channel narrowband stack at each of the gain settings if shot with a F5.6 telescope (which grabs 3-stops or 8x less light than @ F2.0) would be:

Gain 100 (add 3 stops) = 8 x 5 hours = 40 hours integration

Gain 200 (add 3 stops) = 8 x 3 hours = 24 hours integration

Gain 300 (add 3 stops) = 8 x 2 hours = 16 hours integration

That is why I am pleased to have a great F2.0 lens for DSO narrowband imaging.